The global market for light olefins is changing dramatically, both with respect to capacity and with respect to demand. It is estimated that demand in this market will increase by 5% per year until 2010, which requires an increase in production capacity of 5.4% per year in the same period.
At present the two main routes of production of light olefins, such as ethylene and propylene, are pyrolysis (steam cracker) and fluid catalytic cracking (FCC), using conventional units. However, these processes are not meeting the present increase in demand, largely owing to the low yields obtained. Typically, in conventional FCC, the yields for ethylene and propylene obtained are around 0.8% and 5% by weight, respectively. Now, in the pyrolysis process, the yield of ethylene is highly dependent on the feed used, for example if the feed used is ethane, the expected yield is about 70%, but if the feed is light naphtha, the yield drops to somewhere around 30% by weight.
One of the means usually employed for improving the selectivity for light olefins in processes of catalytic cracking, especially FCC, is to change the composition of the feeds processed. It is known that with increase in the size of the carbon chain of olefins and paraffins, their reactivity also increases, and moreover, it is known that olefins are more reactive than paraffins.
U.S. Pat. Nos. 7,375,257 and 6,977,321 describe the production of light olefins by selective cracking of a feed comprising olefins with four or more carbon atoms using zeolites of type MFI as active ingredient of the catalyst.
A process for catalytic cracking of two streams, a main stream rich in paraffins and an additional stream rich in olefins, employing high temperature (500° C. to 700° C.) and low pressure (1 to 30 psia) and a catalyst based on zeolite MFI, has already been described in U.S. Pat. No. 5,043,522. The olefin-rich additional stream is used for compensating the lower reactivity of the paraffin-rich main stream.
Another means of promoting improvement in selectivity for light olefins is modification of the catalysts used in processes of catalytic cracking.
The specialist literature contains various examples of modifications of zeolites that are selective for light olefins, such as ZSM-5, for improving activity, selectivity and stability in FCC processes, such as the patent documents cited below.
U.S. Pat. No. 4,976,847 teaches the use of Pt, Pd, Ni, Co, Fe, W, Mo and mixtures thereof or silicates of Ga, Fe, Sc, Rh and Cr deposited on zeolite ZSM-5, in FCC processes, for maximizing the yield of light olefins.
U.S. Pat. No. 6,153,089 already describes the use of Pt, Pd, W, Mo, Re and mixtures thereof for modifying zeolite ZSM-5, applied to FCC of hydrocarbon feeds, with the aim of producing light olefins and aromatic hydrocarbons.
Documents WO2005094492, WO200669535 and EP 0901688392, describe the use of transition metals, such as Fe, Co, Ni for the modification of zeolite ZSM-5, for direct use or in conjunction with conventional FCC catalysts, so that the resultant catalytic system increases the yield of light olefins in FCC processes for petrochemical raw material—PFCC. These documents deal almost exclusively with the use of iron in the modification of zeolites, as well as the use of feeds that are much more reactive than saturated hydrocarbons of low molecular weight. Furthermore, the modifications carried out on zeolite ZSM-5 are not capable of altering the ethylene/propylene ratio, in terms of selectivity.
Patent application US 2006/0116544 A1 describes the use of Mn or Zr in combination with rare earths and phosphates in type ZSM-5 zeolites. This combination promotes better retention of active sites at high temperature and in the presence of steam. The stability of this catalytic system in pyrolysis processes proved to be superior to that of the processes already known. However, there is no indication regarding selectivity with respect to production of olefins.
U.S. Pat. No. 6,888,038 relates to a method for obtaining olefins by the catalytic cracking of feeds of C4-05 hydrocarbons using a zeolite as catalyst, more specifically a type MTT zeolite, and to the co-processing of a stream comprising an oxygenated hydrocarbon.
Although the use of feeds that are more reactive and modification of the catalysts employed in processes of catalytic cracking have been able to provide a significant increase in selectivity for light olefins, the processes used at present still employ severe operating conditions, especially with regard to the temperatures applied.
For example, in the case of petrochemical fluid catalytic cracking (PFCC), which uses a catalytic system based on zeolites of type ZSM-5, for maximizing propylene, temperatures are applied in the range from 560° C. to 590° C., and cracking of the light hydrocarbons generated (C4-05 olefins) only begins above 600° C., with a consequent increase in the production of ethylene.
In a recent publication, Jiangyin Lu et al. show that a small amount of chromium deposited on zeolite ZSM-5 improves the conversion of isobutane to ethylene and propylene in catalytic cracking processes. However, the operating conditions used are severe, employing temperatures above 600° C. (Catalysis Letters, Vol. 109 (2006) 65-70, “Cr-HZSM-5 zeolites—Highly efficient catalytic cracking of iso-butane to produce light olefins”).
To summarize, a catalytic cracking process that uses a highly active catalyst for cracking reactions of saturated hydrocarbons of low molecular weight and provides, at the same time, greater selectivity for ethylene, in milder reaction conditions, is still unknown.